1. Field of the Invention
This invention relates to process for producing a superconducting transistor using an oxide superconducting material.
2. Description of the Prior Art
In recent years, a proximity-effect-type superconducting transistor has been proposed by, e.g. Japanese scientists Nishino et al in their article entitled "A Superconducting Transistor" in a Japanese periodical publication "Applied Physics" (Vol. 56, No. 6(1987) P 752 to 756).
In a proximity-effect-type transistor, when a superconductor and a normal conductor are in close contact with each other, Cooper pairs induce percolation of charges from the superconductor to the normal conductor. It is possible to make use of a proximity effect for inducing superconductivity on a thin film having a thickness similar to a percolating distance of a normal conductor. That is, when the film thickness is approximately equal to the percolating distance of an SNS junction in which a normal conducting film N is supported between two superconducting films S, Cooper pairs can percolate between superconducting films S due to the proximity effect. The percolating distance is equivalent to the coherence length within a normal conductor. As the temperature is decreased, the percolating distance becomes larger. Further, as the concentration of free electrons is increased, the percolating distance is also increased.
It is difficult to change the concentration of free electrons in a metal. However, in a semiconductor, it is possible to guide a carrier carrying the current to a junction where the concentration of carriers can be changed by providing a field in a manner similar to a field effect transistor.
A proximity-effect-type transistor employs a structure shown in FIG. 16. A source electrode 42 and a drain electrode 43 are formed upon a crystalline silicon plate 41. A gate electrode 44 is formed under the silicon plate 41. Each electrode is a lead alloy superconductor. Numerals 45, 46 and 47 are insulating films.
To produce a conventional proximity-effect-type transistor, the semiconductor and superconductor must be laminated to each other. When forming a high temperature, oxide superconducting thin film upon a semiconductor, the substrate must be heated to more than 800.degree. C. At such temperatures, the superconducting thin film and the semiconductor tend to diffuse into each other. As a result, the properties of the transistor are degraded. For this reason, it has been difficult to laminate, successfully, a high temperature, oxide superconducting thin film with a semiconductor.
There is also proposed a tunnel injection type superconducting transistor which is a known technique. The concept and structure of the tunnel injection type superconducting transistor are disclosed in e.g. IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-21, No. 2 March 1985 "A New Superconducting Base Transistor" P. 721 to 724 or in vol. MAG-19, No. 3, May 1983 "QUITERON" P. 1293 to 1295.
Referring now to FIG. 17, a superconducting transistor comprises a collector area 51 of a semiconductor, a base area 52 of a superconductor thereadjacent and an emitter area 54 of a superconductor. The emitter area 54 of a semiconductor is disposed on base area 52 with a very thin insulating layer 53 between them. Tunnelling may occur through the insulating layer 53.
However, no concrete structure for making the high temperature superconducting transistor of FIG. 17 has yet been proposed. The superconducting transistor is made of Nb material, which requires the super low temperature of liquid helium. None of the above publications teaches a high temperature superconductor that can operate at temperatures attainable with liquid nitrogen.